The present invention relates to a projection system for printing circuit patterns on photoresist coated wafers in semiconductor microlithography. It relates in particular to a class of projection system wherein a photoresist coated silicon wafer is covered with circuit patterns by sequentially exposing different areas of the wafer. Such a system is generally referred to as a step-and-repeat exposure system or simply a stepper.
The present invention relates more particularly to an illumination system for a unit magnification optical projection system for use in microlithography such as described in U.S. Pat. No. 4,964,705 which is assigned to the same assignee as the present invention, and the details of which are hereby incorporated by reference.
That unit magnification projection system is a unit magnification, achromatic, anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion, i.e., a catadioptric optical system, to achieve large field sizes and high numerical apertures. In that system the reticle and wafer are parallel with a window being provided on the reticle adjacent to the patterned area to allow for the projection of the reticle (mask) pattern through the reticle window onto the wafer. The unit magnification system is referred to as a Half-Field Dyson system for reasons which will be explained in detail in the present specification.
To use such a system in a stepper for semiconductor fabrication an accompanying illumination system is required. The above referenced unit magnification system employs a reflective reticle as opposed to transmissive reticles of prior art projection systems. As such, light may not be introduced into the projection system through the reticle. The light is preferably introduced into the system through an optical element of the projection system such that a well defined, uniformly illuminated area may be imaged on to the reflective reticle.
Desirably, the illumination system provides a uniform patch of illumination over the reticle pattern area with no spill over through the adjacent reticle window onto the wafer.
In view of the foregoing, there is a need for an improved illumination system for use with a unit magnification optical projection system.